As a catalyst for fuel cell or a catalyst for automobile exhaust gas, metal catalysts made of metal, particularly noble metal such as platinum are used. Since noble metal element exist on the earth in a limited amount, it is required to decrease the amount to be used as small as possible and to improve the action suited for use as the catalyst as much as possible. Therefore, as the metal catalyst, for example, there can be used those having a structure in which fine particles of metal such as platinum are supported on the surface of carrier particles made of such as carbon black or an inorganic compound.
The catalytic action is mainly exerted on the surface of metal. Therefore, in order to decrease the amount of metal as small as possible while maintaining an excellent catalytic action in the metal catalyst having the structure described above, it is effective to use fine metal particles to be supported on the surface of carrier particles, having a particle diameter as small as possible and a large specific surface area.
Examples of the method for supporting fine metal particles on the surface of the carrier particles include high temperature treating method referred to as an impregnation method, liquid phase reduction method and vapor phase method. Among these methods, there has recently been prevailing the liquid phase reduction method in which manufacturing facilities can be simplified with ease, namely, a method comprising the step of reducing ions of metal to be deposited by the action of a reducing agent in a reaction system of a liquid phase containing carrier particles dispersed therein thereby to deposit the metal on the surface of the carrier particles in the form of fine particles.
In order to decrease the particle diameter of the fine metal particles to be formed by the liquid phase reduction method, it is effective to decrease the rate of reduction and deposition of ions of metal. It is important that a reducing agent having a reducing power as weak as possible is selected and used so as to meet these requirements. Therefore, there is proposed that a metal catalyst is prepared by depositing fine metal particles having a particle diameter as small as possible on the surface of the carrier particles through reduction using a reducing agent having a weak reducing power, for example, alcohol such as ethanol, alkanolamine or ascorbic acid (see, for example, Patent Documents 1 to 3).
In the metal catalyst having a structure in which fine metal particles are supported on the surface of carrier particles, the amount of fine metal particles to be supported and the distance between adjacent fine metal particles on the surface of carrier particles are considered to be important factors which exert an influence on catalytic activity. As the catalyst for fuel cell in which an operating temperature is comparatively low such as 100° C. or lower, such as direct methanol type or solid polymer type catalyst, for example, there can be used a metal catalyst in which fine metal particles such as fine platinum particles are supported on the surface of carbon particles.
However, there is a problem that, when the amount of fine metal particles to be supported is small because the distance between adjacent fine metal particles supported on the surface of carbon particles in the metal catalyst is too large, sufficient catalytic activity can not be obtained. That is, when the surface of exposed carbon widely exists on the pathway between fine metal particles through which oxygen gas or hydrogen gas diffuses, diffusion of the gas is inhibited and thus catalytic activity decreases.
Accordingly, taking account of an enhancement in catalytic activity, the amount of fine metal particles to be supported is preferably as large as possible. However, when the distance between adjacent fine metal particles decreases excessively because the amount of fine metal particles to be supported increases too much, there arises a problem that plural fine metal particles behave like one catalyst particle against a hydrogen gas or an oxygen gas and thus catalytic activity decreases.
Describing in more detail, a region referred to as a territory exists around individual fine metal particles supported on the surface of carrier particles in a size larger than the substantial size. For example, in case of a positive electrode of a fuel cell, an oxygen gas passes through a thin film constituting the territory and reaches the surface of the fine metal particles, where it is subjected to a reductive reaction. However, when the distance between adjacent fine metal particles is too small, territories overlap each other and plural territory compete with each other for one oxygen molecule, and thus catalytic activity decreases.
Namely, the amount of fine metal particles to be supported, and the distance between adjacent fine metal particles on the surface of carrier particles are antinomic to each other. Therefore, Patent Document 4 proposes that, by defining a minimum distance between fine metal particles which is free from overlap of territories, the amount of fine metal particles to be supported and the distance between fine metal particles are well balanced and thus catalytic activity of the metal catalyst is improved.
Patent Document 1: Japanese Unexamined Patent Publication No. JP04-298238A (1992) (claim 1, columns 0003 to 0005)
Patent Document 2: Japanese Unexamined Patent Publication No. JP2000-107606A (columns 0012 to 0013)
Patent Document 3: Japanese Unexamined Patent Publication No. JP2000-279811A (claims 1 and 2, columns 0013 to 0015)
Patent Document 4: Japanese Unexamined Patent Publication No. JP02-065064A (1990) (claims; page 2, upper right column, line 14 to the same page, lower left column, line 18)